The invention relates to a device and method for analyzing atomic and/or molecular elements by means of wavelength dispersive x-ray spectrometric devices, comprising at least a mirror or focussing device with multi-layer structures, particularly a device wherein fluorescent radiation induced by incident primary x-rays or electron beams from a sample to be analyzed is directed No a mirror or focussing device before the radiation reaches a measuring or analysis detector.
Such apparatus and methods are used in scientific analyses, but also in the industry for detecting atomic and/or molecular elements in various applications for example for detecting or analyzing very small amounts of impurities or disturbances in a sample. A particularly important area of application in the industrial field is, for example, the examination of semiconductor wafers (silicon wafers, germanium wafers), which form the basis for the manufacture of highly integrated semiconductor circuits.
In this process, x-ray or electron beams of any type are directed onto a sample whereby, as part of the radiation reflected from the sample fluorescent light is emitted. The fluorescent light is generated by the incident x-rays by known physical processes. Before the fluorescent light beam reaches a measuring or analysis arrangement for example in the form of a fluorescence radiation selective detector, it is directed onto a suitable crystal from which it is reflected onto the measurement and analysis detector. The crystals are effective as analyzers. The crystals can be manufactured artifically and may consist of thin alternating layers of two or more materials with different x-ray optical properties. The incident fluorescent light radiation is reflected by these crystals but only that part of the radiation for which the Bragg equation
nxcex=2d sin "THgr"
is fulfilled. Herein is       λ    ⁢          xe2x80x83        ⁢          (      nm      )        =      1.24          E      ⁢              xe2x80x83            ⁢              (        keV        )            
wherein n=a natural number (n=1,2,3,4 . . . ); xcex is the wavelength of the x-radiation; d is the periodicity (lattice parameter) of the analysis crystal; 2"THgr" is the refraction angle and E is the energy of the x-radiation. If the effects of the refraction are taken into consideration, which effects are very small for x-radiation, a modified equation is obtained from which the wavelength of the reflected x-radiation can be determined with the giver, angles "THgr" and the lattice parameter d of the analyzer based on the first equation or, respectively, the modification thereof. With a variation of the angle, the wavelength of the reflected rays can be selected in a controlled manner.
The advantage of the artificial crystals which, consisting of many uniformly changing layersxe2x80x94so-called multi-layer structures, is that the materials of the multi-layer can be selected so as to optimize the operation. This is an important advantage of the artificially manufactured multi-layer structures as compared to material crystals.
The intensity of the reflected light depends to a large degree on the material used for the multi-layer structures. It is also possible to vary the lattice parameters within wider limits as it is possible with natural crystals.
It is therefore a particular advantage of the multi-layer structure acting as an analyzing device that the analysis of light elements is facilitated with a uniform intensity and without unhealthy side effects. This is an additional advantage when compared with natural crystals, if natural crystals can be used at all for the analysis of light elements.
So far the multi-layer structure or, respectively, the individual layers of the multi-layer structure has been adjusted to the atomic or molecular element that was expected from the sample being examined. Of high importance in the semiconductor industry is, for example, the determination of the boron content in oxygen-containing materials such as boron phosphorus silicate since this material is generally used during the manufacture of microelectronic components.
So far, a multi-layer structure of molybdenum boron carbide layers has been used for the detection of boron. Such a layer is for example described in U.S. Pat. No. 4,785,470.
Such a molybdenum boron carbide multi-layer and the tungsten carbon multi-layers, which have been used for that purpose, have in the energy range of 183 V only a reflectivity of about 35.4% or, respectively, 10% at an optimal angle of "THgr"=26.5xc2x0 (with a tungsten carbon multi-layer structure). Furthermore, the use of tungsten carbon multi-layer structures for the detection of boron in samples, which also contain oxygen, has been found problematic. This is essentially because the emission line of oxygen with a value of E=525 eV has essentially three times the energy of the emission line of boron with E=83 eV. Accordingly, the multi-layer structure reflects in accordance with the equation given earlier, the oxygen line in the third Bragg order (n=3) at about the same angle as the boron line in the first Bragg order (n=1). Since the earlier referred to tungsten-carbon multi-layer has for E=525 eV at "THgr"=26.7xc2x0 in the third order still a reflectivity of 0.24%, a wavelength dispersive separation of the boron and oxygen lines and, consequently, a clear detection of the two elements is insufficient with this multi-layer if at all possible.
The result is improved if molybdenum-boron carbide multi-layers (Moxe2x80x94B4C) are used as they are for an optimum detection of boron in commercial x-ray fluorescence spectrometers. In comparison with a Wxe2x80x94C multi-layer a clearly increased reflectivity of 35.4% in the first Bragg order is achieved. At the same time, the reflectivity of such a Moxe2x80x94B4C multi-layer for 525 eV in the third Bragg order is reduced to 0.16% so that the oxygen line is somewhat suppressed.
It is however a disadvantage that Wxe2x80x94Cxe2x80x94 as well as Moxe2x80x94B4C multi-layers have also a significant reflectivity for E=90 eV. This is also very important for the semiconductor industry since the silicon-L-emission lines are about at 90 eV. Computations reveal that a Moxe2x80x94B4C multi-layer with d=8 nm at an angle of "THgr"=25.9xc2x0 have, in addition to the desirable high reflectivity at E=183 eV for the optimal detection of boron, also an undesirable reflectivity of about 3.2xc2x0 at E=90 eV. This results with boron-containing samples such as boron phosphor silicate (BPSG) disposed on silicon wafers in an increased background signal which is disadvantageous for the x-ray spectrometric detection limit of boron.
It is the object of the present invention to provide a device and method for an improved x-ray analysis for the detection of boron wherein the device and the method can utilize known means and procedures so that available analysis equipment can essentially be continued to be used and the equipment can be easily and inexpensively installed and operated in research laboratories and industrial plants.
In a device and a method for the analysis of atomic and molecular elements by way of wavelength dispersive x-ray spectrometric structures including at least one mirror or focussing device having a multi-layer structure onto which fluorescent radiation generated by primary x-ray or electrons beams from a sample to be examined is directed and the reflected fluorescence radiation is supplied to a measuring device for determining the nature of impurities contained in the sample, the multi-layer structure consists of at least a lanthanum layer and a boron carbide layer.
With the device according to the invention, the detection of boron is greatly facilitated particularly in the energy range of 180 eV. The particularly favorable x-ray optical properties of the materials forming the layer pairs such as lanthanum and boron carbide provide, in comparison with the earlier mentioned known analyses, for an increased reflectivity for the boron line as well as a substantially improved suppression of the oxygen-Kxe2x80x94 as well as the silicon-L-lines.
The multi-layer structure consisting of the base layer pars lanthanum and boron carbide has for the boron line a reflectivity of 60% in the first Bragg order. This is almost twice the value obtained by the best analyzers known up to now. Furthermore, the reflectivity for 90 eV is only 0.65% so that the suppression of the Si-L-line with respect to the earlier solutions is improved by the factor 5. At the same time, at 525 eV, the reflectivity is only 0.016% so that the suppression of the oxygen line is improved by more than a factor 10 over the best results obtained with the best multi-layer structure analyzers presently in use.
The sum of all these factors results in a substantially improved signal-noise ratio and, consequently in a substantial improvement in the x-ray spectrometric detection limits particularly for boron.
In a particularly advantageous embodiment, the multi-layer structure consists of a number of 1 to 100 layer pairs, that is, of 2-200 individual layers. The number of layers or, respectively, layer pairs, which are selected for the formation of a particular multi-layer structure depends essentially on the desired analysis or respectively, measuring task and the expected type and amount of impurities in the sample to be examined.
It is particularly advantageous if the multi-layer structure consists of a number of 40 to 50 layer pairs, that is, of 80 to 100 individual layers.
In a basic version of the device, the thickness of each multi-layer structure is constant; but it is also possible to provide in each multi-layer pair layers with different thickness.
In the embodiments described above, it is basically made sure that a parallel fluorescence beam is reflected over the whole surface of the multi-layer structure with maximum intensity.
In another advantageous embodiment, the thickness of the respective multi-layer structure varies over the area thereof as far as it can be made sure that parallel fluorescence rays, which reach the multi-layer structure under different angles, are reflected over the whole surface area of the multi-layer structure with maximal intensity. The variation of the incident angles "THgr" is compensated for by a variation of the lattice parameter d in accordance with the earlier referred to equation or the computation-corrected modification thereof so that xcex remains constant.
Preferably, the device is so modified that the multi-layer structure is curved. In another advantageous embodiment, she multi-layer structure is disposed on a substrate. However in all embodiments, the multi-layer structure may be disposed on a substrate. It can be made sure in this way that a non-parallel fluorescence beam, which reaches the multi-layer structure at different locations at different incident angles, can be transformed in its beam shape so that for example a divergent fluorescent light beam reaching the multi-layer structure becomes a parallel or a focussed fluorescent light beam. It may also be advantageous to provide for different thicknesses of the individual layers of the multi-layer structure, that is, to modify the thickness of a layer over the extent of the layer so that the multi-layer structure reflects the desired wave length of the fluorescent radiation reaching the multi-layer structure under different incident angles over the surface with maximum intensity.
The multi-layer structure may also have layer arrangements wherein one of a pair of layers has a uniform thickness whereas the other of the pair of layers is of varying thickness.
Preferably, the thickness of the layer is in the area of 1 to 20 nm. Tests have shown that, with those thicknesses, the highest reflectivity and resolution can be obtained for the multi-layer structure.
The method for the analysis of atomic and/or molecular elements by means of wavelength dispersive x-ray spectroscopic devices including a mirror and focussing arrangement with at least one multi-layer structure onto which the primary x-ray or fluorescence rays are directed in such a way that induced fluorescent light generated by a sample as a result of primary x-ray or electron beams directed to a mirror or focussing arrangement before reaching a measuring or analysis detector is characterized in that the primary x-rays or the fluorescent light is directed to a multi-layer structure consisting of at least a layer pair of lanthanum (La layer) and a boron carbide layer (B4C layer).
With the method according to the invention, a reflectivity of over 60% in the first Bragg order can be reached for the boron line. This is almost twice the value reached by the best of today""s methods in which multi-layer structures are employed as analyzers.
Generally, the advantages as listed for the device according to the invention are also provided by the method according to the invention.
The invention will be described below gun greater detail on the basis of an embodiment with reference to the accompanying drawings.